I can play either a cleric or a paladin in the group and with 2 of each I can easily find one that will suit the needs. It doesn't matter who I play so it means I can select my character when we see who else will show up and with what. If we need a healer then I play the cleric, but if someone else heals then I can play the paladin. For hard quests we can use 2 healers and then we have a Favored Soul (Azraelck) and my cleric.

I enjoy being the main healer and I automatically look at the red bars of people so I can heal injuries. I also look at the blue bar so I can give some extra spellpoints. Afflictions is also something I look at. Unfortunately it happens quite often that my healing attempts fail because target is out of range or blocked sight. So I think people who likes to be healed should make sure they're not so far away from the cleric. I try to stand behind the main fighter and support him/her against the main monsters. So those of you who fight wandering in other directions might regret that soon.

For variation I like to be a melee character and then I play paladins. One reason is that I like the boosts to the character, especially the self healing. That means I have emergency heals of people who get incapacitated. That has saved the group many times. So getting many lay on hands is high priority. It also makes it easier to be a cleric because the paladin can do some healing once the cleric mana drops low.

I think the group should always have the following components:
* Rogue (or a bard in not trap heavy dungeons)
* Melee (fighter, monk or paladin)
* Healer (cleric or favored soul)
* Spellcaster (wizard or sorcerer)

Extra character could be anyone, but an extra melee character would be good to support the tank, e. g. a ranger. With both healer and spellcaster we will have a lot of buffs. With a bard we will bathe in buffs. The best use for a rogue is that he/she will find and disable most traps so we won't lose HP's crossing them. Finding secret doors and extra chests will help too. Later the rogue will become so strong so he/she can do backstabbing. At low levels I guess the rogue will fight with ranged weapons.

The sixth character can be anyone, but a second spellcaster is always very nice. Spellcasters are good as long as we have enough melee characters to protect them.

The point is that all characters are useful in DDO and the best groups have a variety of them.